Posted
by
CmdrTaco
on Thursday October 08, 2009 @09:14AM
from the i-can't-believe-i-used-the-transportation-topic dept.

KentuckyFC writes "In 1924, the influential German mathematician David Hilbert calculated that a stationary mass should repel a particle moving towards or away from it at more than half the speed of light (as seen by a distant inertial observer). Now an American physicist has pointed out that the equal and opposite effect should also hold true: that a relativistic particle should repel a stationary mass. This, he says, could form the basis of a 'hypervelocity propulsion drive' for accelerating spacecraft to a good fraction of the speed of light. The idea is that the repulsion allows the relativistic particle to deliver a specific impulse that is greater than its specific momentum, an effect that is analogous to the elastic collision of a heavy mass with a much lighter, stationary mass, from which the lighter mass rebounds with about twice the speed of the heavy mass. Unlike other exotic hyperdrive proposals, this one can be tested using the world's largest particle accelerator, the LHC, which will generate beams of particles with the required energy (abstract). Placing a test mass next to the beam line and measuring the forces on it as the particles pass by should confirm the theory — or scupper it entirely."

The point of the drive is not that it enables light speed, or that it saves energy, because it doesn't do either.

The point of the drive is that it would accelerate you and you *don't* feel it!

The drive would accelerate you by gravity. Just like the International Space Station astronauts are still falling towards the Earth, but they can't feel it- you can't feel relativistic gravity either.

So you could accelerate at 1000 times the Earth's surface gravity if you wanted, and not even spill your coffee (potentially, if it works, and it should do).

Of course scaling up an effect that is only faintly sensed on an accelerator the size of the LHC is left as an exercise to the reader;-), but it's fundamental research and you never know where it could lead.

You could use this to insulate a crew capsule from the effects of acceleration. A more conventional starship could be accelerating at 10G, 100G or even more, and a onboard particle accelerator could be in turn pushing the crew habitat ahead of the starship by this means.

From the traveler's frame of reference, you can travel any distance in any amount of time. I can get from here to Andromeda in 5 seconds, and if I measured my speed relative to light I'd clearly see that I'm travelling at 0% of C the whole time (although I'd see my surroundings flying past me at 99.999% of C or whatever). My measurement of the distance I travelled would also clearly show that I had only travelled a relatively short distance (maybe

It hasn't "worked" fully at all, yet. But it is one of the more complex science instruments on the planet, not a Toyota Pickup truck at the garage. Give them time and it'll do its job... unless some twelve-year old Chinese prodigy figures out a way to do the same stuff in his lunch box.

It hasn't "worked" fully at all, yet. But it is one of the more complex science instruments on the planet, not a Toyota Pickup truck at the garage. Give them time and it'll do its job... unless some twelve-year old Chinese prodigy figures out a way to do the same stuff in his lunch box.

Who would be immediately lynched by the scientific community because no one likes a smart ass.

Where hyperspeed was possible unless there were ships or asteroids nearby. In that case you became "mass locked" So it turns out that more than just a gimmick to skip the boring bits of the game, mass does indeed interfere with fast moving objects.

OK. Me first. Got dibs on Andromeda. Poor chaps what will they do when they discover that we had filed the plans to build a highway through them and taped it to the underside of a sink in an unused bath room in a dark basement guarded by leopards?

So hey, physics dudes... would this work? A space ship that's black on one side and white on the other. The white side reflects light, the black side absorbs it... besides being warmer than the white side would it slowly begin to move? Maybe a millimeter a century or so?:) Long range probes I guess.

The black side would get 1 kick per photon, the reflective side 2 kicks per photon. Net result, 1 kick. A better idea would be a mirror sail that transmits light on one face, and reflects it on the other.

A colleague of mine asked if I thought this was possible or hokum. The authors own "paper" (unpublished preprint, linked above) contains a rather lot of self-references to other unpublished preprints, usually a sign of some level of crack-pottedness.
Also, his own numbers in the abstract for this idea (an acceleration of 3 nm/s^2 for 2 ns) make this completely unworkable. That corresponds to a displacement of a test mass of 1.5 x 10^-35 m. The most sensitive displacement detectors are the laser gravitational wave observatories, each of which are a pair of perpendicular 10km Fabry-Perot cavities. These detectors have a sensitivity of about 10^-18 m. That's seventeen orders of magnitude difference.
On an amusing note, that displacement is actually the same order of magnitude as the "Planck length". I can't help but wonder whether the author engaged in some silly numerology in order to get it to work out that way.

Well, also consider that even deep space isn't completely empty. There are always at least a few hydrogen atoms floating around in even the most remote corners of the universe, as well as lots of photons, background radiation, possibly dark matter and dark energy, and frankly, stuff that we might not even know of yet.

Just like the significant possibility that there's water and other resources on the moon that we once thought was a vast, barren wasteland of nothing useful, with enough research, we may yet f

Somebody hasn't read much Larry Niven. Why take starlight as-is when you can use solar collectors to gather it up and power a laser to drive your sail?

I'm not sure that the maximum velocity is as much a limit as you think, either. Given the time and proper course, so long as you can get above the local escape velocity (which is easier done by stealing momentum from other celestial bodies than by carrying around fuel) you can go somewhere else.

Ok, I don't understand how solar sails are supposed to work, but if light exerts pressure on a sail, then I see how one might wonder if shooting a given candlepower of light out of a bulb might not produce the same amount of force as the light would exert upon hitting a sail. If that is the case though, then there is no need for a sail at all, as all you would be doing would be re-channeling the starlight that hits your craft out the aft end. Would this mean a glass sphere covered in lenses that focus li

You get some momentum from the photons hitting your solar collector. With that done, you can fire up a laser and either aim it at your solar sail, which will reflect the photons in the opposite direction, from which you get some more momentum. Or you can just fire the laser out the back and not take the loss from imperfect reflection. Oh, and you don't need the sail.

I think he means leaving the solar collectors behind and using them to power a laser that

In the solar sail case, you've got a star at the beginning of the journey to provide acceleration and theoretically you're going to venture to another star which should provide the "fuel" for deceleration. In between, I am assuming that you don't lose any speed since you're traveling in a vacuum.

There a any number of viable methods to not carry fuel with them. Ramjets have been mentioned. Electromagnetic sails (or any kind of repulsor of an externally emitted wave/particle). The time-honored method of stealing velocity from other objects (gravity sling-shot).

And, in this case, if we are using a hypervelocity particle to repell a mass, then the particle could, presumably, be fired from a fixed point (Earth) to repel a ship.

The ONLY exception to this is the "solar sail" concept, which relies on an external source of propulsion.

I believe the idea here is to have a particle accelerator in orbit that will be fired past the spacecraft it is accelerating, so it is analogous to a laser-pumped solar sail. It's also best to think of this as a potential tool for accelerating really low-mass instrument packages intended to do fly-bys of nearby stars, which could be scientifically useful.

The rest of your post sounds remarkably like statements by people back in the '70's that we'd never be able to image the disk of even nearby stars, much less discover or image planets around them.

It may be that what the author is proposing is impossible. There are a number of things in the paper that look highly sketchy to me, but GR ain't my field. Even so, while this method of acceleration for interstellar exploration may not work, the one method that is certain not to work is never bothering to try.

There's no limit to how much energy you can put into your propellent though. If you had Sufficiently Advanced Technology you could put a huge particle accellerator on your spaceship and send your exhuast out behind you at 99.999999999% the speed of light which, in fact, gives you a KE > 1/2m*V^2 due to relativistic effects. In fact, you could accelerate a million ton spacecraft up to.5 c with half a kilogram of propellent if you could put enough energy into it.

Nobody said they're getting around anything. In general relativity, such equations balance out only within the context of one inertial frame of reference. The relativistic particle and the stationary mass are in two separate frames of reference. I haven't read the 1924 Hilbert paper, but it sounds like the change in the frame of reference between the relativistic particle and the stationary mass transfers additional KE to the latter.

Agreed mathematically. I see two problems with near-light speeds of travel:

1) You need to get around the mass vs acceleration issue -- I recommend a good warping of space: bring your destination to you.

2) Inertial dampening. To accelerate to near c is either going to take a very long time, or it's going to give someone a pretty severe case of whiplash (as in "Get that unrecognizable pulp out of the captain's chair!"). Braking has the same issue.

That's the classical formula, which is asymptotically accurate at speeds much below the speed of light.

The real formula is messier, as you'll see at the Wikipedia article [wikipedia.org]. There's currently no way around that one, but we might find a more precise formula later.

If the classical formula was completely correct, then the kinetic energy of a particle at lightspeed would be half the relativistic energy of its rest mass, and therefore modern particle accelerators (which can be seen as adding kinetic energy to particles) would achieve speeds far faster than light. This doesn't happen.

There's no inherent limit to the amount of kinetic energy we can put into a particle of any mass. The issue for interstellar travel is getting the energy, not applying it. Energy on that scale has an awful lot of mass.

Agreed. But my point (and maybe not clearly communicated) was that the hyperdrive may be the "break-out" technology which changes the basic premise (liquid and solid fuel rockets). So long as we're chemical based, we're likely limited to wandering (with manned space flight) fairly close to home. Even our next nearest planets are highly impractical for manned flight with current technology.

There is no method of propulsion that is not some manner of standard Newtonian momentum transfer (including the theory in TFA). Even a gravitational assist is a momentum transfer from a planet to a spacecraft--the spacecraft gets a nice speed boost, and the planet is now orbiting a few meters-per-trillion-years more slowly.